Diamond abrasive article and method of making the same



Patented Jan. 24, 1950 DIAMOND ABRASIVE ARTICLE AND METHOD OF MAKING THESAME Reinhold A. Schaefcr, Rocky Hill, Conn., assignor to NortonCompany, Worcester, Mass, a corporation of Massachusetts No Drawing.Application August 3, 1948, Serial No. 42,348. In Canada May 28, 1946 '3Claims. (Cl. 51-408) This invention relates to vitrified-bonded diamendabrasive articles such as grinding wheels in which the abrasive elementcomprises diamond abrasive particles, and to a method of making thesame. This application is a continuation in part of my copendingapplication Serial No. 6%,979, filed .l'uly I3, 1945, and now abandoned.

Among the features and objects of this invention are the provision of avitrifiable bond of a firing or maturing temperature below that at whichoxidation of'the small diamond particles takes place so that the firingof the green" or molded abrasive article can be accomplished withouthaving to resort to firing in special atmospheres, and which vitrifiablebond will have thermadly-responsive expansion characteristics so relatedto those of the diamond abrasive that bonding and holding of the diamondparticles in the final product will not be detrimentally afiected byoperational temperature changes which the grinding wheel undergoes andwhich vltrifiable bond will not detrimentally affect, during firing, theshape and volume of the molded arucle itself. Abrasive grains of a widevariety of natural and artificial substances or materials haveheretofore been used or proposed to be used in the making of abrasivearticles and they range not only from substances relatively soft to thehardest substances known, namely, diamond, but they also range or differwidely in their respective thermal coeficients of expansion, and hencethey are not usually usable interchangeably with the same vitrified bondto produce safe or operative or eficient grinding wheels. For example,where one abrasive material would not produce crazing or like fracturingof the vitrified bond, another, in the same bond, would. Moreover, inamultiple or two-part grinding wheel, the thermal expansivity of theabrasive annulus as a whole would be materially altered by substitutingin it abraslve grains of different thermal expansivity with respect tothe thermal expansivity of the back or support which is not intended tobe abrasive in action, with the result that undue strains or stressescould be created at the junction between the two parts. Another objectof this invention is to provide a method of making a vitrifiedbondeddiamond grinding wheel by which maturing of the bond may be effected attemperatures below the oxidation temperature of the diamond particleswithout giving rise to detrimental or injurious difierences in thermalexpansivities.

Another object is to provide a vitrified bond of relatively softcomposition, capable of strongly holding or bonding the diamond grainsagainst the reactions from their cutting action, and capable ofsubstantially exhausting the cutting capabilities of the diamond grainsand yet give way or itself wear away in a manner to achieve substantialuniformity of wheel wear and thus lessen the frequency of truing duringthe life of the grinding wheel. Another object is to provide a simplerand less expensive method of making vitrified-bonded diamond abrasivearticles, and to provide a product resulting from such method that willhave improved and desirable structural and functional characteristics.Other objects will be in part obvious or in part pointed outhereinafter.

The invention accordingly consists in the features of construction,combinations of elements, arrangements of parts, and in the severalsteps and relation and order of each of said steps to one or more Of theothers thereof, all as will be illustratively described herein, and thescope of the application of which will be indicated in the followingclaims.

I have found that oxidation of diamond particles, when included in a mixof suitable vitrifiable bond, takes place during firing in an atmosphereof air at temperatures above a value of 750 0.; when the maturingtemperature of the bond is above that value, the effects of oxidation ofthe diamond particles become evident in various ways and to varyingextents. For example, microscopic examination shows discoloration of thebond structure, apparently by by-products resulting from the oxidationor alteration of the diamond, and then again the effects of attack uponor reaction with the diamond sometimes appear in the form of bloating ofthe vitrified article, due probably to the evolution of gases resultingfrom the reaction with the carbon of the diamond.

In accordance with my invention I am enabled to avoid or materiallydiminish such undesirable efiects as those above noted, without havingto resort to expedients such as the incorporation in the mix of easilyoxidized ingredients and/or the use of a non-oxidizing atmosphere andthus I am enabled greatly to simplify the production of vitrified-bondeddiamond abrasive articles, and also to achieve certain other desirableadvantages. According to my invention I have discovered that I can firethe molded mix of diamond particles and bond ingredients in an oxidizingatmosphere such as air, without material risk of oxidation of thediamond particles. This unexpected result I am enabled to achieve, inpart, by first making a up a glass or glasses to have certaincharacteristics later set forth, and to such glass or glasses I then addother ingredients, including diamond abrasive particles for the abrasivepart, to make up a mix out of which the abrasive article is then moldedand is then fired in the manner later described. According to myinvention I provide a range of glasses to meet various requirementswhich glasses can be matured without; recourse to such expedients asthose above mentioned. A number of examples are hereinafter set forth.

As one illustration of a glass accordin to my invention, I make up a mixof silica and lead oxide, in suitable proportions and under conditionsand treatment which are later set forth. This glass may be characterizedas a lead silicate glass and it may contain alkali metal oxide. Theingredients of the mix, to which may be added other ingredients latermentioned by way of illustration, are thoroughly mixed together, heatedto a relatively high temperature, preferably to about 1300 C. but at allevents above the final maturing temperature of the article, underconditions later described, and held at such a temperature until theingredients are melted together. Upon subsequent cooling the glassyproduct thus formed is crushed to particle sizes very small, on theorder of 100 grit size or smaller. The resultant glass frit, dependingupon what other ingredients are initially added to the mix, has asoftening or maturing point at or below 750 C. and other characteristicssuitable to functioning according to my invention. Whenever I refer tosoftening" herein, for good bonding. scribed, to provide diamond bondingtermined softening can provide a frit or glassy bond that may be maturedduring firing at a temperature on the order of 600 C. in an oxidizingatmosphere and this is particularly advantageous where the diamond grainis exceedingly fine and thus is more susceptible to oxidation under heattreatment. I also may provide a glass bond that may be matured at asomewhat higher temperature, for example, up to 750 0., and such a bondmay be employed advantageously particularly for diamond in larger gritsizes having less tendency to be oxidized in an oxidizing atmosphere.

In making up the frits I prefer to avoid the existence or production offree lead in the glass; hence it is preferable that the lead oxidesemployed be devoid of free lead. For example, I may preliminarily treatthe lead oxide as by heating it in an oxidizing atmosphere to insure theconversion of any free lead to lead oxide. Any of the lead oxides may beemployed such as PbO although it is preferred to employ the more stablehigher oxides such as Pb3O4.

In making up the mixes for the frits and heating the mix to brin theingredients thereof into solution as above described, it is usuallydesirable I mean softening, sufllciently I am enabled, as is laterdefrits of suitable or desirable characteristics with predeto preventthe lead oxide ingredient from reducing to free lead, and desirably alsothe heating of the mix is for similar reasons carried on under oxidizingconditions as, for example, in an oxidizing atmosphere such as air. Theinclusion of an oxidizing agent in the mixes for the frits may beeffected by including alkali metal oxide in the form of a nitrate whichunder heat treatment of the mix furnishes oxygen to insure the oxidationof any free lead that might be initially present and also inhibits thereduction of the lead oxide to free lead.

Also, in making up the glass frit, it is desirtemperatures; for example,I

able, during the heating cycle thereof, to achieve substantial releasetherefrom of gases or vapors so as substantially to avoid the productionor evolution of gases during the subsequent firing of the vitrifiablebond and abrasive mix and an illustrative heating cycle may compriseheating the ingredients of which the frit is to be made to about 1300C., in for example, a gas-fired furnace, under oxidizing conditions asabove noted because of the lead ingredients, and maintainingthattemperature until all of the batch is in solution and a clear melt isobtained, whereupon the temperature may be dropped to about 1100 C. andheld at that temperature for a sufficient time to allow for theevolution and escape of the gases from the melt;-the melt may then bequenched in water or be poured onto a suitable slab, allowed to cool,and then crushed and ground to the desired comminuted form such as 100grit size or smaller.

9f the lead silicate glasses that may be made up according to myinvention, for making up the frit, an illustrative composition mayconsist of lead oxide (either PbO or Pb304) about 70% by weight, silicaabout 27%, and alkali metal oxide such as sodium oxide or potassiumoxide about 3%. The latter ingredient may, however, be omitted; it isincluded in any case in small proportion and primarily for the purposeof contributing its good fiuxing qualities and thus better initiatin thegoing into solution of the several ingredients when the mix is heated tomake up the frit.

Using this frit, a two-zone grinding wheel may be made up with a centeror back comprising a mix, by volume, of 50% of this frit and 35% of 100grit size vitreous silica, molded under pressure and other conditions togive a volume porosity of about 15%, and having an abrasive annuluscomprising by volume 50% of this frit, 18.6% of 100 grit size diamond,and 16.4% of vitreous silica of 100 grit size, molded under pressure andother conditions to give a porosity of about 15%; the two mixes forthese parts are made up with the aid of a suitable plasticizer orbinding agent appropriate to give the composite wheel structure whenmolded and pressed appropriate green strength so that it may uponremoval from the mold be handled and trued or shaved as desired and thensubjected to firing. The molding is preferably carried on undersubstantial pressure, on the order for example of from 20 to 25 tons persquare inch. The molded "green wheel may be dried in an oven, forexample, at C., overnight, and then may be fired at about 660 C. in air,bringing the temperature up to that value over a period of about fourhours, maintaining it at the 660 C. for about two hours, and thenallowing it to cool slowly. Upon comparative test, the wheel wasexceptionally soft-acting when grinding cemented tungsten carbide.

In the firing of the above described wheel, no detrimental swelling orslumping took place, the firing temperature being approximately suitedto the above described range of softening temperatures of the fritemployed; moreover, no evidence of harm to the diamond grain was presenteven though the firing took place in an oxidizing atmosphere. Harmfulgas evolution appeared to be absent and in this connection a factor liesvery probably in the manner of preparation of the frit as abovedescribed, substantial expulsion of gases therefrom being adequatelyprovided for. Moreover. I am also enabled to preclude harmful effectsduring firing by the employment of a temporary binder, in making up thegreen" wheels. that insures the absence of residues capable of causingharmful eifects diu'ing firing. I prefer to employ inorganic temporarybinders such as magnesium bentonite which contains calcium carbonate andwhich I, therefore, preliminarily acidify with hydrochloric acid so asto get rid of the Co: which the carbonate would produceduring heattreatment. The magnesium bentonite comes in the form of a powder. Thisis stirred with water and then acidified with hydrochloric acid. Itshould then be allowed to stand for a number of hours whereupon it formsa gel. Then the supernatant liquid is decanted.

A small quantity, on the order of 1% or so, of this inorganic gel Iemploy in making up the mix of frit, body material, and abrasiveparticles, for making up and molding the green" wheel. Also,- I mayemploy silica gel which is silicic acid (HzSiOa) or alumina gel which isAl(OH):; these gels are also inorganic. Unlike organic bindersheretofore employed, such as glutoline, glycerine, etc., etc., and whichupon firing leave some residue, usually of carbon which can becomeoxidized into CO2 during firing in an oxidizing atmosphere or in thepresence of oxidizing ingredients or entrapped air in the green wheeland thus causing bloating, swelling or the like, the inorganic temporarybinders that I may employ leave no residual material which could form abloating gas and they are otherwise not harmfully reactable duringfiring. The molded green wheels, utilizing these temporary inorganicbinders, are before firing preferabl dried in an oven. for example, at90 C. for ten hours or so, and these temporary binders break down intowater which is expelled as a vapor during the drying and firing, andinto substances which, during the firing, react with the glass of thebond composition or are dissolved in it. For example, the acidifiedmagnesium bentonite gel breaks down into water, magnesium silicate, S102and some C8012 which react with or are dissolved in the glass; thesilica gel breaks up into water and silica and the alumina gel breaks upinto water and A1203. I may, however, also use plain water as atemporary plasticizing agent for molding the "green wheel, the waterbeing driven oif during the oven drying that precedes the firing. Therelative proportions of the silica and lead oxide may be widely varied,with or without additional ingredients of which the alkali metal oxidesuch as sodium oxide is an illustration; the silica may be present inpercentages by weight from to 60% and the lead oxide from to 70%. Fritsso made up have softening points broadly in the neighborhood of 600 C.as, for example, within a range from about 500 C. to 750 C. Ingredientslike alumina may be included in small percentages in order to make theglass of the frit and hence of the ultimate bond less soft. The leadsilicate frit glasses have thermal coemcients of expansion, being on theorder of from x1o" to 85X 10-". When used as the glassy element of thevitrifiable bond, th molded or green abrasive article may be fired attemperatures appropriate to the soltening temperature of the particularfrit and hence within a general range of from 500 C. to 750 0., and itwill be noted that these maturing temperatures are below deleterioustemperatures as above described. I

I may raise the softening point as by adding a mall percentage, on theorder of 3% by weight,

6 of oxide (mo); in that manner I may raise the softening point andsubstantially correspondingly elevate the maturing temperature by about100 C., but in any case the maturing temperature should be not aboveabout 750 C.

with any of these lead silicate frit glasses I may compound the mixesfor the abrasive part of the abrasive article and for thenon-diamondcontaining support or backing part thereof, by

the addition of other ingredients appropriately coactive, particularlyphysically, with the glassy bond and, in the abrasive part, with thediamond abrasive particles, particularly in the light of relativethermal expansivities. For example, I prefer to employ, as a bodymaterial in either part, although it may be omitted from thediamondcontaining part, a comminuted body material having a thermalcoeiiicient of expansion as closely ating that of diamond as possible.The expansion coefllcient of diamond is about 113x10 and illustrativeand suitable body materials to employ are vitreous silica or a highlyrefractory glass, preferably in grit sizes of or smaller. These have thecharacteristic of softening points for bonding greatly in excess of thematuring temperature of the bond and they do not go into solution orotherwise react with the glass of the bond in a deleterious mannet andinstead are bonded by the latter as individual body material particles.In relation to the lead glasses of my invention, they respond totemperature changes without detrimentally affooting the glassy bond asby fracturing or crazing it or the like. They may be employedindividual- 1y or in admixture.

As an example of a highly refractory glass suitable for use as abovedescribed, I may use a well known glass which has approximately thefollowing formula: 80.6% $102, 13.0% B203, 3.8% NazO, 0.4% K20, 2.2%A1203. This glass does not soften at 750 C.

As above mentioned, the lead silicate glasses of the bond of myinvention are relatively soft; nevertheless they strongly hold and bondthe diamond particles and in this action they are aided by the includedbody materials such as vitreous silica, the particles of highlyrefractory glass, etc. Although the glass element of the bond structureis relatively soft, particularly as compared to vitrified bondsheretofore employed in diamond abrasive articles, the cutting action isfreer than in the hard-bonded dense vitrifiedbonded diamond abrasivearticles heretofore employed. With its softness, the glass element of mybond structure appears to have also a quality of give or of relativeyielding, which may be advantageous in avoiding breakage of wheels underhigh speed grinding conditions.

It will thus be seen that there has been provided in this invention adiamond abrasive article and method of producing the same in which thevarious objects above mentioned together with many thoroughly practicaladvantages are successfully achieved. The method of my invention will beseen to be simpler and less costly to carry on than methods heretoforeemployed and the diamond abrasive article will be seen to be highlyadvantageous in construction and action. Risk of harming the propertiesof the diamond grain which, of course, can be employed in variousamounts or concentrations is practically absent.

As many possible embodiments may be made of the above invention and asmany changes might be made in the embodiment above set forth, it is tobe understood that all matter hereinbefore set forth is to beinterpreted as illustrative and not in a limiting sense.

I claim:

1. A diamond abrasive article comprising diamond grains free fromdetrimental oxidation held in a substantially unswelled vitrified bondstructure that comprises pre-fired lead silicate glass consistingessentially of by weight silica 25% to 60% and lead oxide 40% to 70% thearticle being vitrified at and its bond having a maturing temperaturebetween 500 C. and 750 C. and said glass having been made by prefiringat a temperature above that employed in maturing the article, and saidarticle having a volume precentage of porosity not greater than 15volume percent.

2. Method of making a vitrified bonded diamond abrasive article whichcomprises mixing materials containing silica and lead oxide, heating themixture under oxidizing conditions to a temperature high enough toproduce a glass, cooling the glass and crushing the glass to produce apowdered frit, the ingredients and the proportions of the ingredients ofthe mixture being such that the frit consists essentially of by weightsilica 25% to 60% and lead oxide 40% to 70% and the frit has a softeningpoint between 500 C. and 750 C., mixing diamonds with said frit, moldingsaid diamonds and frit into a shaped article, and vitrifying said shapedarticle at a maturing temperature between said 500 C. and said 750 C.

3. Method according to claim 2 in which the CJI 8 materials that areheated to produce the glass and then crushed to produce the frit alsocontain alkali oxide but in proportion less than either of the silicaand the lead oxide.

4. Method according to claim 2 in which the vitrifying is done underoxidizing conditions.

5. A diamond abrasive article according to claim 1 having a centercomprising refractory glass bonded with the same unswelled vitrifiedbond that bonds the diamond grains.

6. Diamond abrasive article according to claim 1 also containingcomminuted vitreous silica as an ingredient which becomes incorporatedinto the fired structure.

7. Diamond abrasive article according to claim 1 also containingcomminuted highly refractory glass as an ingredient which becomesincorporated into the fired structure.

REINHOLD A. SCI-IAEFER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,338,598 Thomas Apr. 27, 19201,529,259 Locke et a1 Mar. 10, 1925 2,018,817 Taylor Oct. 29, 19352,281,526 Milligan et al Apr. 28, 1942 2,334,266 Houchins Nov. 16, 19432,343,218 Lombard Feb. 29, 1944

1. A DIAMOND ABRASIVE ARTICLE COMPRISING DIAMOND GRAINS FREE FROMDETRIMENTAL OXIDATION HELD IN A SUBSTANTIALLY UNSWELLED VITRIFIED BONDSTRUCTURE THAT COMPRISES PRE-FIRED LEAD SILICATE GLASS CONSISTINGESSENTIALLY OF BY WEIGHT SILICA 25% TO 60% AND LEAD OXIDE 40% TO 70% THEARTICLE BEING VITRIFIED AT AND ITS BOND HAVING A MATURING TEMPERATUREBETWEEN 500*C. AND 750*C. AND SAID GLASS HAVING BEEN MADE BY PREFIRINGAT A TEMPERATURE ABOVE THAT EMPLOYED IN MATURING THE ARTICLE, AND SAIDARTICLE HAVING A VOLUME PRECENTAGE OF POROSITY NOT GREATER THAN 15VOLUME PERCENT.